1. Field of the Invention
This invention relates to a process for producing a lithium manganese oxide which is suitable for use as a positive electrode or a cathode material of a non-aqueous electrolyte (organic electrolyte) lithium ion secondary or rechargeable battery. More particularly, this invention relates to a process for producing a lithium manganese oxide with a spinel structure which is suitable for use as a cathode material of a lithium ion secondary battery having a 4-V high energy density and excellent cycle performance by a simple process using less costly starting materials.
2. Prior Art
A lithium cobalt oxide, LiCoO2 so far has been used as a high performance cathode material of a non-aqueous electrolyte lithium ion secondary battery. However, since the amount of cobalt from natural sources is small and it is expensive, a lithium manganese oxide with a spinel structure, namely, spinel LiMn2O4, has attracted attention as a cathode material of a non-aqueous electrolyte lithium ion secondary battery in the next generation.
In the non-aqueous electrolyte lithium ion secondary battery in which the spinel LiMn2O4 is used as a cathode material, lithium ions can be doped in the crystal lattice or removed therefrom in the charge or discharge process, with the result that the charge or the discharge stably can be conducted without greatly destroying the structure of the crystal lattice. Thus, the study for practical use of the spinel LiMn2O4 in the non-aqueous electrolyte lithium ion secondary battery has been conducted.
In a known dry method for producing spinel LiMn2O4, as described in Japanese Patent Application Laid-Open (Kokai)No. 4-198028, a powder of manganese dioxide and a powder of a lithium compound are mechanically dry-mixed directly, and the resulting powder mixture is calcined at a high temperature.
However, it is difficult to obtain a uniform powder mixture having a particle diameter of a submicron level by mechanically mixing manganese dioxide with a lithium compound, while the thermal conductivity of these metal oxides is quite low. Accordingly, in order to obtain spinel LiMn2O4 by calcining such a powder mixture, it is required that as described in Japanese Patent Application Laid-Open (Kokai) No. 7-78611 the powder mixture is calcined at high temperatures over a long period of from 10 to 100 hours and the calcination at such high temperatures is repeated. Besides, the initial charge-discharge capacity of the thus obtained cathode material falls far short of the theoretical value (148 mAh/g). When the charge-discharge process of the non-aqueous electrolyte lithium ion secondary battery which uses such a spinel LiMn2O4 as a cathode material therein is repeated, the charge-discharge capacity of the battery is notably deteriorated.
Meanwhile, a process which comprises dissolving manganese acetate and lithium acetate in ethylene glycol by heating, then removing the solvent to form a gel-like mixture, and calcining the mixture to provide spinel LiMn2O4 is described in Japanese Patent Application Laid-Open (Kokai) No. 6-203834. In this process, however, the starting materials used are costly, and it can hardly be employed as an industrial process for producing spinel LiMn2O4.
In general, a solid state reaction using two solid powders as reactants is carried out by heating the solid powders at high temperatures capable of moving ions or atoms constituting the reactants, and mutually diffusing the ions or the atoms between the solid phases of these two solid powders. Since the two solid powders cannot be mixed uniformly at the molecular level in this solid state reaction, the solid state reaction starts where the solid phases are brought into contact, and the reaction product is formed in the boundary therebetween. In order that the reaction product is formed in the boundary between the two solid phases and the reaction further proceeds, it is necessary to conduct the movement of the materials in which the ions or the atoms in at least one of the two solid phases pass through the layer of the reaction product and are diffused into the other solid phase. Accordingly, the smaller the particle diameter, the shorter the diffusion distance and the larger the diffusion surface. Thus, a dense product having a uniform composition easily can be obtained.
When spinel LiMn2O4 is produced by the dry method, it is necessary to mix manganese dioxide and a lithium compound as starting materials. In this case, manganese dioxide has a particle diameter of from several microns to tens of microns. Meanwhile, lithium hydroxide has a particle diameter which is approximately 100 times larger For this reason, it is impossible to mix these two compounds uniformly. Further, as noted above, metal oxides such as manganese dioxide and lithium manganate have a low thermal conductivity; when these compounds are heated to hundreds of degrees Celsius, it takes a few hours to make uniform the temperatures of the sample from the surface to the inner portion.
Thus, when the calcination temperature and time are inappropriate in the calcination of the mixture of the solid powders as starting materials at a high temperature to produce spinel LiMn2O4 by the conventional dry method, there arise temperature differences in the mixture, and the elements are locally concentrated. Consequently, products other than final spinel LiMn2O4 are formed. In particular, when the calcination temperature is too high, not only does the crystallization of the resulting lithium manganese oxide proceed excessively, but also undesirable by-products are formed, with the result that the performane of the resultant cell is adversely affected.
That is, in the formation of spinel LiMn2O4, it is important that the starting solid powders are uniformly mixed and that the mixture of the solid powders is uniformly heated at a high temperature to proceed rapidly with the desired solid state reaction.
Under these circumstances, the present inventors have assiduously conducted the formation of spinel LiMn2O4, and have consequently found that a gel-like uniform mixture of manganese dioxide and lithium compound can easily be obtained by mixing manganese dioxide with a lithium compound selected from lithium hydroxide and lithium carbonate, preferably lithium hydroxide, in a solvent which is selected from water, an alcohol and a mixture thereof, preferably either an alcohol or a mixture of the alcohol and water, to dissolve the lithium compound in the solvent, and diffusing the resulting lithium ions into pores of porous particles of manganese dioxide. This mixture is calcined preferably through combination of heating with microwave and with an electric furnace, making it possible to obtain desired spinel LiMn2O4 easily and efficiently. This finding has led to the completion of this invention.
As set forth above, this invention has been made to solve the problems in the conventional production of spinel LiMn2O4. It is an object of the invention to provide a process for producing a lithium manganese oxide with a spinel structure which is suitable for use as a cathode material having a high energy density and excellent cycle performance at high efficiency by a simple process using less costly starting materials. In particular, it is an object of the invention to provide a process in which a uniform gel-like mixture of manganese dioxide and a lithium compound easily can be obtained and spinel LiMn2O4 having excellent performace easily can be obtained by calcination of the gel-like mixture for a short period of time.
A first process for producing a lithium manganese oxide with a spinel structure according to the invention comprises mixing at least one lithium compound selected from the group consisting of lithium hydroxide and lithium carbonate with manganese dioxide in a solvent consisting essentially of water or an aliphatic lower alcohol having from 1 to 3 carbon atoms or a mixture of these, allowing the resutant mixture to form a gel-like mixture, drying the gel-like mixture as required, and calcining the resulting product.
A second process according to the invention comprises mixing at least one lithium compound selected from the group consisting of lithium hydroxide and lithium carbonate with manganese dioxide in a solvent consisting essentially of water or an aliphatic lower alcohol having from 1 to 3 carbon atoms or a mixture of these, in the presence of an organic acid selected from the group consisting of formic acid and acetic acid, allowing the resutant mixture to form a gel-like mixture, drying the gel-like mixture as required, and calcining the resulting product.
Futher according to the invention, in both of the first and second processes, it is advantageous that at least a part of heating for drying and calcining of the gel-like mixture is conducted through a combination of heating with microwave and heating with an electric furnace so that spinel LiMn2O4 suitable for use as a cathode material of a lithium ion secondary battery having excellent cycle performance can be produced readily at high efficiency.